using System;

namespace Algorithms.Strings
{
    /// <summary>
    ///     <para>
    ///         Jaro Similarity measures how similar two strings are.
    ///         Result is between 0 and 1 where 0 represnts that there is no similarity between strings and 1 represents equal strings.
    ///         Time complexity is O(a*b) where a is the length of the first string and b is the length of the second string.
    ///     </para>
    ///     <para>
    ///         Wikipedia: https://en.wikipedia.org/wiki/Jaro%E2%80%93Winkler_distance#Jaro_similarity.
    ///     </para>
    /// </summary>
    public static class JaroSimilarity
    {
        /// <summary>
        ///     Calculates Jaro Similarity between two strings.
        /// </summary>
        /// <param name="s1">First string.</param>
        /// <param name="s2">Second string.</param>
        public static double Calculate(string s1, string s2)
        {
            if (s1 == s2)
            {
                return 1;
            }

            var longerString = s1.Length > s2.Length ? s1 : s2;
            var shorterString = s1.Length < s2.Length ? s1 : s2;

            // will look for matching characters in this range
            var matchingCharacterRange = Math.Max((longerString.Length / 2) - 1, 0);
            var matches = 0d;

            // true if i-th index of s1 was matched in s2
            var s1MatchedIndeces = new bool[s1.Length];

            // true if i-th index of s2 was matched in s1
            var s2MatchedIndeces = new bool[s2.Length];

            for (var i = 0; i < longerString.Length; i++)
            {
                var startIndex = Math.Max(i - matchingCharacterRange, 0);
                var endIndex = Math.Min(i + matchingCharacterRange, shorterString.Length - 1);
                for (var j = startIndex; j <= endIndex; j++)
                {
                    if (s1[i] == s2[j] && !s2MatchedIndeces[j])
                    {
                        matches++;
                        s1MatchedIndeces[i] = true;
                        s2MatchedIndeces[j] = true;
                        break;
                    }
                }
            }

            if (matches == 0)
            {
                return 0;
            }

            var transpositions = CalculateTranspositions(s1, s2, s1MatchedIndeces, s2MatchedIndeces);

            return ((matches / s1.Length) + (matches / s2.Length) + ((matches - transpositions) / matches)) / 3;
        }

        /// <summary>
        ///     Calculates number of matched characters that are not in the right order.
        /// </summary>
        private static int CalculateTranspositions(string s1, string s2, bool[] s1MatchedIndeces, bool[] s2MatchedIndeces)
        {
            var transpositions = 0;
            var s2Index = 0;
            for (var s1Index = 0; s1Index < s1.Length; s1Index++)
            {
                if (s1MatchedIndeces[s1Index])
                {
                    while (!s2MatchedIndeces[s2Index])
                    {
                        s2Index++;
                    }

                    if (s1[s1Index] != s2[s2Index])
                    {
                        transpositions++;
                    }

                    s2Index++;
                }
            }

            transpositions /= 2;
            return transpositions;
        }
    }
}
